In recent years, the development and practical application of LED drive circuits for driving LEDs are advancing. An example thereof is LED illumination employing PWM constant current control. FIG. 1 is a circuit diagram illustrating the configuration of an LED drive circuit according to a related art. The LED drive circuit includes, as illustrated in FIG. 1, a regenerative diode D1, a reactor L1, and a driver IC 1 to drive an LED 10. The driver IC 1 incorporates a switch element Q1 such as an FET, a drive circuit 3 of the switch element Q1, and a constant current circuit 2. The driver IC 1 is connected to an external current detection resistor R1 that is connected in series with the switch element Q1. The current detection resistor R1 may be incorporated in the driver IC 1. The constant current circuit 2 has a comparator 21 and a control circuit 22, to prevent a current equal to or over a given value from passing to the switch element Q1.
The comparator 21 compares a reference voltage Vref inputted to an inverting terminal (depicted by “−”) with a voltage generated by the current detection resistor R1 and inputted to a non-inverting terminal (depicted by “+”) and outputs a comparison result to the control circuit 22. The current detection resistor R1 is connected in series with the switch element Q1, generates, at both ends thereof, a voltage corresponding to a current passing to the switch element Q1, and applies the voltage to the non-inverting terminal (+) of the comparator 21. According to the comparison result of the comparator 21, the controller 22 outputs a signal to adjust an ON duty ratio of the switch element Q.
According to the output signal from the control circuit 22 in the constant current circuit 2, the drive circuit 3 of the switch element Q1 applies a voltage to a gate of the switch element Q1, thereby turning on/off the switch element Q1.
Operation of the LED drive circuit according to the related art is explained. FIG. 2 is a view explaining operation of the LED drive circuit according to the related art. FIG. 3 is a view illustrating operating waveforms at various parts of the LED drive circuit according to the related art. In FIG. 2, the constant current circuit 2 and drive circuit 3 of the switch element Q1 are not illustrated but they are actually present.
First, the switch element Q1 turns on at time t0 of FIG. 3 when a predetermined gate voltage Vg is applied from the drive circuit 3 of the switch element Q1. At this time, an ON current (load current) passes as illustrated in FIG. 2 through a path extending along a power source through the reactor L1, switch element Q1, and current detection resistor R1 to the ground, thereby accumulating counter electromotive force in the reactor L1.
Between time t0 and t1 of FIG. 3, the ON current increases at a predetermined inclination (di=L1×dV/dt) according to a constant of the reactor L1. The ON current passes through the current detection resistor R1, and therefore, a voltage Vrs generated by the current detection resistor R1 also increases.
At time t1, the voltage Vrs generated by the current detection resistor R1 exceeds the reference voltage Vref, and therefore, an output from the comparator 21 inverts. Then, the control circuit 22 outputs a signal to turn off the switch element Q1. According to the output signal from the control circuit 22, the drive circuit 3 of the switch element Q1 decreases the gate voltage of the switch element Q1, to turn off the switch element Q1.
As a result, energy of the counter electromotive force accumulated in the reactor L1 passes as a regenerative current through a loop extending along the reactor L1, LED 10, and diode D1 and is thereby consumed. Accordingly, no current passes through the current detection resistor R1, and therefore, the voltage Vrs across the current detection resistor R1 becomes zero.
After the elapse of a predetermined time, the control circuit 22 outputs at time t2 a signal to turn on the switch element Q1. Namely, the LED drive circuit according to the related art explained here is a circuit that carries out an OFF time fixed operation. According to the output signal from the control circuit 22, the drive circuit 3 of the switch element Q1 increases the gate voltage of the switch element Q1, to turn on the switch element Q1.
The LED drive circuit according to the related art illustrated in FIG. 1 repeats the above-mentioned operation, to supply a proper current to the LED 10 and drive the same.
Japanese Unexamined Patent Application Publication No. 2006-319221 (Patent Document 1) describes an LED drive circuit that is realizable with a simple structure and passes an equal current to a plurality of parallel LED circuits. This LED drive circuit includes a current source that generates a temporally changing current and first and second smoothing capacitors. The LED drive circuit drives a first LED circuit that is arranged in parallel with the first smoothing capacitor and has one or a plurality of LEDs connected in series and a second LED circuit that is arranged in parallel with the second smoothing capacitor and has one or a plurality of LEDs connected in series. Further, the LED drive circuit includes a current dividing coil that has two coils connected to each other through a tap to which the current generated by the current source is passed, a first reverse current protection diode connected between a first end of the current dividing coil and a first electrode of the first smoothing capacitor, and a second reverse current protection diode connected between a second end of the current dividing coil and a first electrode of the second smoothing capacitor.
According to this LED drive circuit, the temporally changing current generated by the current source passes through the current dividing coil. At this time, the current dividing coil causes an electromagnetic coupling action to divide the current of the current source into currents according to an inverse ratio of two numbers of turns without regard to forward current-forward voltage characteristics of the first and second LED circuits. The divided currents are passed through the respective reverse current protection diodes to the parallel smoothing capacitors and LED circuits. With this, required currents (for example, equal currents) pass through the respective LED circuits even if the LED circuits have different forward current-forward voltage characteristics. This solves problems such as an uneven light quantity (brightness) between the LED circuits, a temperature increase due to different current values, and a difference between service lives and provides high-quality products of simple structure. This also reduces manufacturing costs.